The present invention relates to a method for welding and for forming a chromium oxide passivated film at the welded portion, and also to an apparatus for performing the method. More particularly, it relates to an improved method and apparatus for welding and for forming a chromium oxide passivated film at the welded portion with improved reliability and repeatability.
There are many circumstances in which it is highly desirable to produce vessels, conduits and other fabricated parts with a continuous, uninterrupted passivated surface film on the interior and/or exterior surface. A passivated surface film provides a relatively inert protective layer on material handling equipment and processing equipment where high purity and low contamination are important. Areas of technology and manufacturing where these are important include production of pharmaceuticals and medical devices and fabrication of high vacuum equipment, such as is used for production of semiconductor devices and superconducting electronic devices. However, when such equipment is fabricated by welding using conventional methods, the welded portions may create an interruption in the passivated surface film. It would be highly desirable in these circumstances to provide a method and apparatus for creating welded joints that have a passivated surface film on the welded portion. It would be even more desirable to provide a method and apparatus for producing the passivated surface film simultaneously with creating the welded joints with high consistency and reliability. Such a method and apparatus would be particularly applicable to the welding of stainless steel, in particular ferritic stainless steels, where it is desirable to produce welded joints having a chromium oxide (Cr2O3) passivated film at the welded portions.
In the electronic industry, the pace of development of semiconductor devices having high LSI and high performance has increased greatly, and manufacturing apparatuses which are capable of producing such semiconductor devices have been in demand, and energetic efforts have been made to produce atmospheres having a higher degree of vacuum and higher cleanliness. High vacuum apparatuses of this sort are also in demand for other semiconductor manufacturing, superconductor thin film manufacturing, magnetic thin film manufacturing, metallic thin film manufacturing, dielectric thin film manufacturing, or the like; for example, film formation apparatuses or processing apparatuses employing sputtering, vapor deposition, CVD, PCVD, MOCVD, MBE, dry etching, ion implantation, diffusion and oxidation furnaces, or the like, or furthermore, evaluation apparatuses employing Auger electron spectroscopy, XPS, SIMS, RHEED, TREX, or the like.
In order to produce atmospheres having an ultra-high degree of vacuum and having ultra-high cleanliness, it is necessary to completely suppress the release of gas from the apparatus as well as from the gas supply piping system. Previous research efforts in this area have resulted in the development of a chromium oxide passivated film treatment method, so that it has become possible to form, on a surface, an oxide passivated film having chromium oxide as a chief component thereof, which is corrosion resistant and non-catalytic, and which furthermore restricts gas release in the extreme. These previous efforts have succeeded in producing an atmosphere in which the release of gas from the internal portions of the apparatus is restricted to an amount which is difficult to detect with present measuring devices.
However, as such apparatuses have become larger in scale, and as the complexity thereof has increased, it has become increasingly necessary to connect pipes or apparatuses by means of welding after the formation of the chromium oxide passivated film. Because the surface of the welded portions is not covered by a chromium oxide passivated film, gas easily adsorbs thereto and desorbs therefrom, and as the number of welds increases, the gas which is released therefrom reaches a level which is impossible to ignore, so that a new problem is generated in that the atmosphere becomes contaminated.
Furthermore, in apparatuses or piping systems employing corrosive gases or the like, there are problems in that the welded portions are corroded, and as a result of this, the atmosphere becomes contaminated. The formation of an oxide passivated film in apparatuses or piping systems which are large-scale and have complicated forms requires a complex passivated film formation apparatus, and in particular in view of increasing costs, a welding method in which a passivated film can be formed simultaneously with welding has been strongly desired.
U.S. Pat. No. 5,597,109 granted to Tadahiro Ohmi, Shinji Miyoshi and Yasumitsu Mizuguchi describes a welding method for forming a chromium oxide passivated film at a welded portion, including a welding apparatus and process apparatus. The specification of this patent is hereby incorporated by reference in its entirety. In the welding process disclosed, a back shielding gas having a moisture content of 800 ppm (parts per million) to 2.5% is used to encourage formation of a thick passivated film of chromium oxide at the welded portion. While this previous patent represents a significant advance in this technical area, continued research has been directed toward further improvements of such a welding process, in particular toward the goals of ease of operation of the welding system and higher reliability and repeatability of the welding process and of the welded joints produced.
In view of the above points, the present invention has as an object thereof to provide an improved welding and passivation method which is capable of forming an oxide passivated film at the welded portion during the welding process, which is corrosion resistant and non-catalytic and which adsorbs and desorbs extremely little gas at the welded portions and in the heat-affected-zone (HAZ) in the vicinity of the welded portion. In particular, it is an object to provide such a welding and passivation method with easier operation and with higher predictability, reliability and repeatability. Furthermore, the present invention has as a further object thereof to provide a welding and passivation apparatus which is capable of carrying out the improved method.
In a first aspect of the invention, the welding and passivation method of the present invention includes a welding step, in which a welded area is formed on a substrate, and a passivation step, in which an oxide passivated film is formed on the welded area. Typically, the welding and passivation method will also include a shielding step, in which a protective shield of inert gas is formed around the area to be welded and maintained throughout the welding and passivation steps. The shielding step is initiated by flowing a backshield gas, such as ultrahigh purity argon gas, around the area to be welded to purge contaminants and to form a protective shield of inert gas. In the welding step, an arc gas or welding gas, such as ultrahigh purity argon gas mixed with 5-10% hydrogen, is flowed onto the area to be welded and the area is heated to form a welded joint. Next, in the passivation step, the heat is reduced and the welded area is maintained at a reactive temperature for forming an oxide passivated film and a process gas or passivating gas, such as ultrahigh purity argon gas mixed with 30-100 ppm oxygen, is flowed over the welded area to form an oxide passivated film. After an acceptable oxide passivated film has been formed on the welded area, the process gas is turned off and the backshield gas flows into the area to protect and to help cool the oxide passivated welded area.
In a preferred embodiment, the welding and passivation method of the present invention takes the form of a high speed tungsten-inert gas (TIG) orbital tube welding process. In this embodiment of the invention, the method is used to form oxide passivated welded joints in ferritic stainless steel tubing. In the shielding step, a backshield gas comprising an inert gas, preferably ultrahigh purity argon, is flowed over the welding site to purge contaminants and to create a protective atmosphere around the welding site. In the welding step an arc gas comprising an inert gas mixed with a gaseous reducing agent, preferably ultrahigh purity argon mixed with 5-10% hydrogen is flowed through the high speed TIG orbital welding head and a welding current is applied to produce a welded joint in the tubing. Preferably, the arc gas source is connected directly to the high speed weld head, without passing through the welding power source in order to reduce contamination and to maintain the moisture content at less than approximately 1 ppm. In the passivation step, a process gas that comprises an inert gas mixed with a gaseous oxidizing agent, preferably argon mixed with oxygen at a concentration of approximately 30-100 ppm and having a moisture content of less than approximately 1 ppm, and more preferably less than approximately 1 ppb, is flowed over the welded portion to form a chromium oxide passivated film on the welded portion. Preferably, the process gas is passed through a gas purifier just prior to being flowed over the welded portion to maintain its moisture level at less than approximately 1 ppb. Preferably, when the welding system is not in use, it is continually purged with an inert gas, such as argon, to reduce impurities and contamination. When performed according to this improved method, the welding and passivation process of the present invention reliably and repeatably forms a welded joint with a consistent chromium oxide passivated film having a high chromium oxide content over the welded portion.
A second aspect of the present invention is embodied in a welding and passivation apparatus for carrying out the improved welding method as described herein. In a preferred embodiment, the welding and passivation apparatus is characterized as having a welding head, such as a high speed tungsten-inert gas (TIG) orbital welding head or the like, and a gas supply system equipped to deliver the appropriate backshield gas, arc gas and process gas with the required compositions and with the proper timing during the welding process. The gas supply system has associated with it a source of backshield gas, a source of arc gas and a source of process gas. Each of these gas sources is connected to the welding head by gas supply lines made of metallic and/or polymeric tubing. An electronic timing unit and solenoid assembly control an arc gas valve and a process gas valve to release each of the gases into the welding head at the proper time during the welding process. Preferably, the process gas source also includes a gas purifier that the process gas is passed through just prior to being flowed over the welded portion in order to maintain its moisture level at less than approximately 1 ppb. Optionally, the process gas source may also include a moisture concentration monitor for process control. When operated according to the improved welding and passivation method as described herein, the welding and passivation apparatus of the present invention is capable of reliably and repeatably forming welded joints having a chromium oxide passivated film over the welded portion.